Lock mechanisms which perform the function described have long been known in the art. For example, U.S. Pat. No. 679,385 issued July 30, 1901 in the name of Klemme et al. ("Klemme") discloses such a mechanism comprising a hook pivotally coupled at its upper end to a fly rail of the ladder and, at its lower end, having an upside down "U" recess defined between downwardly extending prongs of the hook which are inner and outer, respectively, relative to the base section of the ladder. A keeper lever is pivotally coupled to the lower end of the outer prong and extends therefrom to an upwardly tilted keeper tip. The keeper is weighted to normally close the opening of the hook's "U" recess in such manner that the tip projects beyond the hook's inner prong. That prong is adapted by engaging the inner side of a base rung during downsliding of the fly section to be wedged outwardly to pass by the rung.
With the inner prong and the tip engaging the inner and under sides, respectively, of a base rung of the ladder, a slight lifting of the fly rail will pivot the keeper away from the "U" recess opening and, also, raise the hook's inner prong above the base rung, thereby to permit the hook to swing under gravity about its pivot to position the "U" recess over such rung. With the recess so positioned, subsequent lowering of the rail will bring such rung into contact with the hook wall bounding the upper portion of the "U" recess to shift the weight of the fly section through the hook and contacted base rung to the ladder's base section to thereby preclude further downward movement of the fly section. If, prior to such slight lifting, the fly section is moved up one or more rung interspaces, the hook will pass by one or more higher rungs (by outward wedging of the inner prong) and then may be locked to one of them.
If, on the other hand, the fly rail is lifted enough from the starting position described above to cause the keeper tip to pass by and disengage with the base rung, then the keeper pivots upwards to bear against the hook's inner prong and close the hook's recess, and, on subsequent lowering of the fly rail, the keeper tip will engage the top of the underlying base rung to be wedged outwardly thereby so that the keeper pivots the hook to displace its inner prong outward to the point where the rung will pass by both such prong and the keeper. The hook then pivots back inwardly under gravity to position the keeper tip over the next lower base rung. Continued lowering of the fly section produces reiteration of the hook and keeper actions just described so that the hook and keeper may be passed by a plurality of base rungs.
The Klemme hook-and-keeper arrangement has, with variants in structure, been widely adopted and is in common use in present day extension ladders. Such hook-and-keeper arrangements require however that the pivot for the hook be located at a substantial distance above the adjacent lower fly rung on the fly rail, and this leads to the disadvantage (among others) that the hook must be large in size to the point where the arrangement is bulky, tends to get in the way of the ladder user and may require omission of a fly rung from the fly section in order to mount the arrangement on a fly rail.